1. Field of the Invention
The invention relates to processes for the purification of diaryl carbonates such as diphenyl carbonate.
2. Brief Description of the Prior Art
Diaryl carbonates such as diphenyl carbonate may be prepared by a number of processes. For example, one process is based on the phosgenation of aromatic hydroxy compounds in the presence of a quaternary ammonium salt catalyst. A variant process comprises phosgenation at elevated temperatures in alkaline solution. A commercial process for production of diphenyl carbonate comprises phosgenation of phenol in the presence of caustic at a pH of 10-11 and at a temperature of 55.degree.-60.degree. C.
Commercial preparations of the diaryl carbonates, regardless of the process employed, inevitably contain various contaminant compounds in varying quantities. Examples of contaminants which have been identified are inorganic and organic chlorides, metal ions, iron compounds, acidic compounds such as aryl chloroformates and a range of compounds identified only as "color bodies". These contaminants frequently effect use of the diaryl carbonates in particular applications. For example, when their desired use is as a monomer reactant in the preparation of aromatic polycarbonate resins by transesterification (using bisphenol-A and diphenyl carbonate), presence of the contaminants can affect polymerization rates and resin color. The polymer product may have a low intrinsic viscosity (IV) and colors ranging from pink (iron contamination) to brown (phenyl chloroformate contamination).
Although distillation of diaryl carbonates has been useful to remove color bodies and solves the problem of coloration in the carbonate and resins prepared from them, the procedure has not been useful to remove other contaminants responsible for inhibiting polymerization rates. In fact, a major drawback of distilled diphenyl carbonate is a loss of reactivity as observed in ester interchange reaction studies. The reason is open to speculation. Also, subjecting the diaryl carbonate, such as diphenyl carbonate, to distillation solely as a means of purification results in a percentage of product loss, which is an economic disadvantage.
We have found that relatively crude diaryl carbonate containing contaminants associated with the process of preparation may be purified advantageously by a two-stage procedure to optimize overall yield of the desired product and remove contaminants. The first stage is washing a melt of the diaryl carbonate with water; followed by a second distillation stage. An intermediate cut taken during distillation between the first and last (product) cuts helps to isolate color bodies and contaminants responsible for producing low intrinsic viscosity and/or colored polymer resins therefrom.
The distilled product is of improved reactivity and high purity, when the distillation procedure is immediately preceded by a hot aqueous wash in accordance with the method of the invention. Water washing of the crude diaryl carbonate helps reduce the content of contaminants including iron, and improves final color in polymers prepared from the purified diaryl carbonate.
The process of the invention may be carried out to purify diaryl carbonates in a continuous or a batch procedure, with an advantage in that the product is consistently obtained in high purity.